This invention relates to a new class of polysilanebased polymers useful as photoresists.
U.S. Pat. Nos. 4,588,801 and 4,587,205 and Zeigler et al, "Self-Developing Polysilane Deep-UV Resists-Photochemistry, Photophysics, and Submicron Lithography", SPIE, 539, 166, 1985, describe a novel class of silicon backbone polymers, polysilanes, which, on exposure to light in the deep UV spectral range rapidly depolymerize and form gaseous siloxanes which volatilize from the surface leaving a positive image. These materials are, thus, "self-developing" and, unlike current resists, require no post-exposure solvent development step to produce an image in the resist. The elimination of this solvent development step constitutes a major advance over current resist technology and leads to lower VLSI fabrication costs stemming from process simplification and lower defect rates in the finished IC's.
Promising as these materials may be, they are not ideally suited to all applications. Their main disadvantage is that they are relatively opaque in the deep UV, with extinction coefficients, at 248 nm of 1000-3000 l/cm.sup.2. mole, typically. While this opacity is not a problem for self-developed exposures, it is a problem when the materials are used in a conventional solvent-developed mode. Since most of the light is absorbed in these materials in the top of 0.1 .mu.m of a 1 .mu.m thick film, the top part of the film must be considerably overexposed to allow radiation to penetrate to the bottom of the film. This leads to degradation of resolution and low apparent sensitivity in the solvent developed mode. For effective use as solvent-developed resists, it would be desirable to reduce the value of the extinction coefficient in polysilanes at the exposure wavelength by about a factor of 10.
Another type of silicon-containing polymer is the class of the organopolysiloxanes. See, e.g., U.S. Pat. Nos. 4,247,674, 3,532,731, 3,903,047, 3,706,591, 3,414,597, 3,321,501, 2,998,440, 2,956,045 and 4,273,420. Such polymers contain repeating units of --O--Si--O--. They contain no Si-Si bonds in the main chain as required in polysilanes. Thus, they have essentially no main chain scission photochemistry although organopolysiloxanes are known which potentially might have side chain photochemistry (see U.S. Pat. No. 4,391,963). Since main chain scission photochemistry is an absolute requisite for the use of polysilanes as photoresists, such prior art polymers are of no value.
U.S. Pat. No. 4,358,576 describes another class of Si-containing polymers. However, the main chains of these contain the bonds --CH.sub.2 --Si--O--V--. Such polymers also are not photosensitive.
U.S. Pat. No. 2,696,480 does disclose polymers containing --Si--Si-- bonds, as well as --Si--O--Si--O-- linkages. However, this patent also requires --C.sub.6 H.sub.4 --Si--C.sub.6 H.sub.4 --Si-- linkages. The solid-state photochemistry of such polymers is not described.
Trefonas et al (Journal of Polymer Science: Polymer Chemistry Edition, Vol. 23, 2099-2107 (1985)) discloses organogermane homopolymers and copolymers containing both germanium and silicon in the main chain. The side groups are chosen from n-butyl, n-hexyl, methyl, cyclohexyl and phenyl. It is very generically stated in this document that such polymers may find the use as photoresists.